Add StreamController.multiplex constructor.

pkg/mdv_observe/lib/src/observable_list.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of mdv_observe;
 
 /**
  * Represents an observable list of model values. If any items are added,
  * removed, or replaced, then observers that are listening to [changes]
  * will be notified.
@@ skipping 52 matching lines @@
     }
 
     _list.length = value;
   }
 
   E operator [](int index) => _list[index];
 
   void operator []=(int index, E value) {
     var oldValue = _list[index];
     if (hasObservers) {
-      _recordChange(new ListChangeRecord(index, addedCount: 1, removedCount: 1));
+      _recordChange(new ListChangeRecord(index, addedCount: 1,
+                                                removedCount: 1));
     }
     _list[index] = value;
   }
 
   // The following methods are here so that we can provide nice change events.
 
   void setAll(int index, Iterable<E> iterable) {
     if (iterable is! List && iterable is! Set) {
       iterable = iterable.toList();
     }
@@ skipping 195 matching lines @@
       }
 
       offset += removed - added;
     }
   }
 }
 
 // TODO(jmesserly): bogus type to workaround spurious VM bug with generic base
 // class and mixins.
 abstract class _ListBaseWorkaround extends ListBase<dynamic> {}